The invention relates to a continuous bulk polymerization process for making acrylate polymers suitable for use as flow control additives in systems which are used in the making of thermoset coated substrates, particularly for coating and casting resins, more particularly for powder coating systems, as well as casting resins for potting and flooring applications curing at ambient temperature. Such acrylate polymers can be solvent-free or substantially solvent-free and substantially free of unreacted monomer. In a particular embodiment, the invention relates to a process for making poly(butylacrylate-co-2-ethylhexylacrylate) resins with a low weight average molecular weight, which are suitable for use as flow control additives.
Flow modifiers (i.e. flow control additives) perform many functions in a coating. Flow additives are essential ingredients of many organic resin systems for coating and casting applications. They are described in, for example, L. J. Calbo, Ed., Handbook of Coating additives, Vol. 1, p. 119 et seq., Marcel Decker, New York (1987) and in U. Zorll, Ed., ROEMPP-Lexikonxe2x80x94Lacke und Druckfarben, p. 602 et seq., Georg Thieme Verll, Stuttgart (1998). They are primarily used to reduce or eliminate surface defects, such as craters, fisheyes, orange peel and pinholes. This is achieved by enhancing the wet-out, flow and leveling of the uncured film. Most of the surface defects develop during the application of the coating material on the substrate.
Surface cratering results from insufficient wetting of the substrate by the wet or molten coating material. In order to achieve good wetting, a liquid coating must have a surface tension equal to or lower than that of the substrate. High solids coating systems, such as oil-free polyester/melamines, wet poorly due to the high surface tension of the resins and the use of polar solvents. Cratering also results from contamination of the substrate or the wet film with low surface tension material, such as silicones, greasy dust, or solvent droplets.
The driving force behind the formation of a crater is the flow of material from areas of low surface tension to areas of higher surface tension. Flow modifiers exhibit a surface tension much lower than the resin vehicles, promoting substrate wetting. The polymer structure of the flow modifier defines its surface activity and controls or limits the compatibility of the product in a coating.
Historically speaking, during the latter portion of the 1950s thermoset-type powder coating materials were introduced and used to coat metallic substrates. They generally consisted of a simple epoxy material. The end product was considered a functional, not a decorative, coating. Thermoset-type materials are materials that, when applied to a substrate and heated to a curing temperature, melt, flow and then cross-link chemically. Once cured, this material, if reheated, will not remelt or reflow. As time went by, thermoset-type coatings were applied to substrates to provide both protection and decorative appeal. Surface defects in thermoset coatings were to be avoided not only because they detracted from the appearance of the coatings but also because they could compromise the integrity of the substrate. Early on the presence of solvents in epoxy powder coating systems helped avoid surface defects.
Epoxy coating systems can be liquid systems or in other cases can be powder systems. Epoxy powder coating systems are generally made in a three-stage continuous process as follows. In the first stage, epoxy resins, argumented with other resin, preservatives, dyes, pigments, curing agent and so forth are dry-mixed in a blender. The blended material is then fed into a kneader. Because of the tremendous mechanical energy released inside the kneader, any solid resins quickly melt. Molten material which is extruded from the kneader is then cooled and subsequently pulverized.
Recent advances in coating technology have included the development of high solids, low volatile organic compound (VOC) coating systems and of powder coating systems. The low VOC content (i.e. solvent content) reduces the ability of the coating system to overcome poor wetting and flow at the time when the coating system is heated and cured. Further, vehicles (i.e. thinners) which have been developed for these coatings systems often exhibit poor wetting and flow characteristics, increasing the frequency of surface defects. These trends have resulted in a greater reliance on flow modifiers such as polyacrylates to provide better flow and leveling qualities.
Copolymerized acrylate resins such as poly(butylacrylate-co-2-ethylhexylacrylate) resins have been used as flow control agents for epoxy coating systems. These prior art polyacrylate resins are available in the marketplace. For example, these polyacrylate resins are available from Monsanto, The Chemical Group, 800 N. Lindbergh Boulevard, St. Louis, Mo. 63167 and are sold under trademarks such as Modaflow(copyright), and Modaflow(copyright) 2100. Other such resins are available from Henkel KgaA, Duesseldorf, Germany, or from Henkel Corporation, Ambler, Pa., under the marks Perenol(copyright) F40, Perenol(copyright) F45, and Perenol(copyright)F30P. However, prior art polyacrylate resins have weight average molecular weights in range of 10,000-30,000, which means they are quite viscous and therefore tend to inhibit the flow of coatings systems containing them, sometimes requiring the use of high boiling, diluting carrier oils. This is particularly true if the epoxy coating systems themselves are liquid and are innately thick and/or lack clarity. Also, some of these prior art polyacrylate flow control resins often contain solvents such as xylene which are classified as volatile organic compounds (VOC""s). Such resins when thermoset in ovens generate fumes of VOC""s that are hazardous to work with.
All quantities stated below, except in the Examples, are to be considered modified by xe2x80x9caboutxe2x80x9d. Unless otherwise stated all parts are by weight.
The invention relates to a bulk polymerization process as described in U.S. patent application Ser. No. 08/948,714 filed Oct. 10, 1997, which is a continuation-in-part of U.S. patent application Ser. No. 08/686,860 filed Jul. 26, 1996, both of which applications are incorporated herein by reference. The process comprises the steps of: charging into a continuous tube reactor (CTR) a feedstock of at least one vinylic monomer and a polymerization initiator; maintaining a flow rate through the reactor sufficient to provide a residence time of the feedstock in the reactor of from about one minute to about one hour; while maintaining a pressure in the tube reactor of about 80 psig to about 500 psig and while maintaining the temperature of the resin mixture that forms in the tube reactor, preferably with a heat transfer medium within the range from about 150xc2x0 C. to about 260xc2x0 C.; and then devolatilizing the resin product which exits the reactor to thereby remove unreacted monomers and any other volatiles. An additional embodiment comprises the additional step of recycling the unreacted monomers recovered during the devolatilization step and charging them into the continuous tube reactor as a fraction of the feedstock.
Generally, the invention relates to a process for producing a polymer or copolymer from monomer material comprising an acrylate, methacrylate, or mixture of such monomers, which comprises the steps of:
(a) charging into a continuous tube reactor a feedstock comprising said monomer material and a polymerization initiator;
(b) maintaining a flow rate of said material through the reactor at a reaction temperature in the reactor and under pressure sufficient to provide a residence time of the feedstock in the reactor during which polymerization will occur, to form a resin product in the reactor, and said resulting resin product comprising unreacted monomer, and
(c) devolatilizing said resin product exiting the reactor to remove unreacted monomers, to provide a substantially monomer-free resin product. Further, the invention relates to resin product made by the above process. In particular, the invention relates to resin that has a weight average molecular weight of 10,000 or less, and a glass transition temperature of less than 0xc2x0 C., preferably xe2x88x9220xc2x0 C. or lower.
The invention further relates to using the bulk polymerization process for preparing poly(butylacrylate-co-2-ethylhexylacrylate), and comprises the steps of:
(a) charging into a continuous tube reactor, feedstock comprising butylacrylate, 2-ethylhexyl acrylate, and a polymerization initiator;
(b) maintaining a flow rate through the reactor sufficient to provide a residence time of the feedstock in the reactor of from about 2 min. to about 10 min.;
(c) maintaining a reactor pressure of about 80 psig to about 200 psig;
(d) maintaining the resulting resin at a temperature within the range from about 100xc2x0 C. to about 300xc2x0 C., preferably with a heat transfer medium, to form a resin product comprising poly(butylacrylate-co-2-ethylhexylacrylate) and unreacted monomer; and
(e) devolatilizing the resin product exiting the reactor to remove unreacted monomers to provide a poly(butylacrylate-co-2-ethylhexylacrylate) resin product, whereby said resin is capable of forming clear coatings when used in combination with epoxy coating compositions.
The process further comprises the use of ditertiary butyl peroxide or ditertiary amyl peroxide or tert-butyl hydroxperoxide as the polymerization initiator.
In another embodiment, the bulk polymerization process further comprises an additional step of recycling the unreacted monomers recovered during the devolatilization step and charging them into the continuous tube reactor as a part of the feedstock.
Another embodiment of the invention relates to the product produced by the inventive process. Yet another embodiment the invention relates to coated articles of manufacture made using the products of the invention.
Other embodiments relate to the preparation and use of solvent-free poly(butylacrylate-co-2-ethyl hexylacrylate).
It is contemplated that the resin products of the invention may be used as flow modulators for liquid coating systems based on epoxy, urethane, acrylic, alkyd, phenolic, polyester, melamine, polyamide, silicone rubber, alkyl, EVA copolymers, and cellulosic resins. It is contemplated that the resins of the invention may be used as modifiers for powder coating systems such as epoxy, hybrid, acrylic, polyester TGIC, polyester urethane, and polyester hydroxyl alkyl amide coating systems, particularly cycloaliphatic cured liquid epoxy resin coating systems.
In the process of the invention, monomers are polymerized using a single-pass flow-through tubular reactor. A monomer or a monomer blend and a polymerization initiator blend are separately introduced and then combined via stainless steel tubing. Prior to combination, the monomer or monomer blend may be preheated by pumping it through a preheating section of tubing which is dipped into an oil bath set for a preselected temperature. The preheating ensures that the temperature of the monomer blend will be increased to a desired initiation temperature level prior to entering the tubular reactor. The preheating step is not essential to the process. The combined flows then enter a static mixer where the two streams are homogeneously mixed. At this point, a small amount of interaction may occur if the monomer blend is preheated.
After exiting the static mixer, the combined flows enter a tubular reactor. The reactor consists of a single tube or a series of tubes of increasing diameter bound in a coil, for single pass use. The tubes are plain with no static mixer or other mixing elements therein or in combination therewith after the combined flows enter the tubular reactor, although the reactor tubes can be provided with static or other mixing elements as well. The coil is preferably immersed in a circulating oil bath that is maintained at a preset desired temperature. Temperature sensors placed along the length of the tubular reactor may be provided to monitor the temperature. Initiation and polymerization begin as the combined flows enter the tubular reactor. Conversion is high and the reaction is essentially complete. Unexpectedly, the single-pass flow-through tubular reactor will efficiently accomplish the desired result under the stated conditions.
A particular reactor that may be used is constructed of five 20 foot lengths of {fraction (1/2+L )} inch outside diameter (O.D.) tube, three lengths of 20 foot {fraction (3/4+L )} inch O.D. tube, and two lengths of 1 inch O.D. tube, all 18 gauge 316 stainless steel. They are joined in series and preferably are contained in a shell that is 21 feet long and 8 inches in diameter which contains recirculating hot oil as the heat transfer medium.
The reactor""s design details are not particularly critical, and the reactor size can be scaled up or down within limits. Laboratory scale reactors will work. However, the back pressure of the reactor is sensitive to the tube diameter, length and roughness, the number and radii of the connections as well as to the changing rheological properties of the reaction mixture as it is converted to polymer as it travels the length of the tubing. These are computationally intractable and the optimal pressure control for each reactor design must be developed experimentally as the conversion rate, as will be seen, is a strong function of the pressure in a continuous tube reactor (CTR). The minimum pressure, which is about 80 psig, should be higher than the vapor pressure of the monomer material at the heating oil temperature. The maximum pressure will depend on the hoop strength of the tubing used. The upper pressure limit will also be determined by economics and by heat transfer factors. It may be reasonable to expect this maximum pressure to be about 500 psig. For the reactor described, the optimal pressure range is from about 100 psig to about 300 psig. In this range, the conversion rate can vary from 60% to 99%.
The lower bound for the reaction temperature is about 100xc2x0 C. while the upper bound is about 300xc2x0 C. At lower temperatures, conversion is so slow that residence times become uneconomically long and the viscosities are too high to handle. A preferred temperature range for this reactor and for the monomer/initiator mixture is from about 140xc2x0 C. to about 290xc2x0 C.; more preferred is about 150xc2x0 C. to about 260xc2x0 C.; and even more preferred is about 210xc2x0 C. to about 250xc2x0 C. It may reasonably be expected that a longer tube will require lower temperature for equal conversion, while larger O.D. or thicker walls might necessitate higher temperatures. When the heat transfer fluid is set to 204xc2x0 C. (401xc2x0 F.), the stream at the reactor exit can be as high as 288xc2x0 C. (550xc2x0 F.).
The residence time lower limit is about 1 minute, conversion being low. On the upper end there are diminishing returns on percent conversion as well as economic waste for needless dwell time; this upper time limit is about 1 hour. Also, polymer properties suffer at higher residence times. The preferred dwell time for this reactor is optimized simultaneously with the pressure and temperature, as described above, and is typically about 2 to 10 minutes, preferably 3 to 6 minutes, and more preferably 150 seconds to about 250 seconds.
The process of the invention can be used to make resins with a weight average molecular weight in the range of 10,000-20,000. However, preferably the resins should have a weight average molecular weight of less than 10,000, more preferably less than 8,000, and most preferably less than 5,000. The resins of the invention should have a Tg less than 0xc2x0 C., more preferably a Tg less than xe2x88x9220xc2x0 C., and most preferably a Tg less than xe2x88x9230xc2x0 C. In general, the most desirable resins have a Tg in the range from xe2x88x9230xc2x0 C. to xe2x88x9260xc2x0 C.
While no solvent is required, solvent can, of course, be added.
The feedstock can comprise, for example, butyl acrylate (BA) and 2-ethylhexylacrylate (2-EHA). The composition range of BA to 2-EHA of interest is 75 wt. % to 85 wt. % BA and 15-25 wt. % 2-EHA. A preferred charge that minimizes the formation of gel sphere beads is 77.5 wt. % BA: 22.5 wt. % 2-EHA: 0.43 wt. % di-tertbutylperoxide.
Recycling of the monomers recovered from the reaction mass exiting the reactor as distillate from the devolatilization step is one useful feature of the invention. Typically, about 5 wt. % of the feedstock can consist of recycled monomer. The recycled monomer may require pre-processing such as purification.
The polymerization initiator is of the free radical type with a half-life ranging from about 1 hour to about 10 hours at about 90xc2x0 C. to about 100xc2x0 C. Preferred are initiators with half-lives of about 10 hours at about 100xc2x0 C. Initiators of this sort may be azo-type, such as azo-bis isobutyronitrile (AIBN), 1-tert-amylazo-1-cyanocyclohexane, and 1-tert-butylazo-1-cyanocyclohexane. They may also be peroxides and hydroperoxides such as tert-butylperoctoate, tert-butylperbenzoate, cumene hydroperoxide, dicumyl peroxide, and tert-butyl hydroperoxide. Two preferred initiators are di-tert-butyl peroxide and di-tert-amyl peroxide. The quantity of initiator typically used ranges from 0.01 wt. % to 5 wt. % and preferably 0.1-1 wt. % based on total monomer. When di-tert-butyl peroxide is used it is preferred that it is at about 0.4 wt. %. An even more preferred initiator is tert-butyl hydroperoxide (tert-BHP).
When the reaction product exits the CTR, and is no longer under pressure, the hot acrylic resin will devolatilize and the end product will therefore essentially be volatile free. The cooled product can be used as a flow control additive in epoxy coating systems. The epoxy coating systems may also be augmented with preservatives, dyes, pigments, thixotropes, wetting agents and the like prior to use.
The following Examples explore variations of the reaction parameters, particularly pressure, variations on the percent conversion (one-pass yield), and the properties of the acrylic resins produced. It is desirable to have less than 3% residual monomer in the acrylic resin produced, preferably less than 1%, and most preferably less than 0.5%. If the residual monomer in the acrylic resin is too high, coating systems containing the acrylic resin may not be clear.
All percentages are weight percentages and all molecular weights are weight average molecular weights. The examples that follow relate to copolymers of butyl acetate and 2-ethylhexylacrylate. However, it is contemplated that the product of the invention may be based on a single monomer or on two or more different monomers provided that each monomer has a glass transition temperature (Tg) of less than xe2x88x9220xc2x0 C.
It is further contemplated that the monomers may be selected from a group which includes, for example, ethyl acrylate, hydroxyethyl acrylate, 2-ethyl hexyl acrylate, hydroxyethlyl acrylate, acrylic acid, and methacrylic acid. While the examples below and previous descriptions of the invention often relate to the production of copolymers, the process of the invention is also useful for the production of homopolymers, terpolymers, etc. Acrylic monomer that include long chains having acrylic and/or methacrylic terminals are also useful, but long chains are not preferred for use in coating compositions.